Echo suppressor apparatus



Sept. 21, 1937. D. MITCHELL 2,093,525

' ECHO SUPPRESSOR APPARATUS Filed Oct. 25, 1935 2 Sheets-Sheet 1 FIG.

AW I LW //v l/EN TOR 0. M/ TCHE L L ATTORNEY Patented Sept. 21, 1937 PATENT OFFICE Ecno SUPPRESSOR APPARATUS Doren Mitchell, Bound Brook, N. J., assignor to American Telephone and Telegraph Company,

a corporation of New York Application October 25, 1935, -.,Serial No. 46,801

Claims.

This invention relates to two-way signaling systems and particularly to the signal-controlled 1 circuits employed for directionally controlling signal transmission in such systems.

An object of the invention is to improve the operation characteristics of long two-way signal transmission systems utilizing therein signal-operated apparatus for suppressing echoes.

In two-way telephone signaling systems in 1 which two paths of a long four-wire circuit are adapted for transmission in opposite directions, it has been found necessary in the past to employ voice-operated switching apparatus for disabling one of the paths while transmission is being eil5: fected over the other in order to obtain the best transmission results. Such apparatus serves to prevent echoes of the transmitted voice signals being carried back to the transmitting end of the system and causing a disturbance. The disabling 1 apparatus usually comprises an amplifier-rectifier control circuit connected to each of the two one-way paths of the four-wire circuit, so as to be operably responsive to the initiation of signal transmission therein to disable the other path,

251 the associated control circuit, or both, in any suitable manner. In certain circuits of the prior art, the voice-operated switching apparatus, or echo suppressors as they are called, are located at the terminal ends of the four-wire circuit.

The invention is specifically directed to improvements in such terminal echo suppressors,

the improvements consisting in the provision of means for facilitating break-ins by a listening party after the other party has obtained control of the system, and for minimizing transmission lock-outs, without introducing excessive echo difficulties. This is accomplished mainly by special circuit arrangements producing a suitable differential action between the two suppressor units associated with the incoming and outgoing oneway circuit at each terminal of the four-wire circuit. 7

A more complete understanding of the invention with its various objects and features will be had from the following detailed description when read in connection with the accompanying drawings in which:

Figure 1 is a diagrammatic representation of a four-wire telephone transmission circuit equipped with voice-operated switching apparatus embodying the invention; and

' Figs. 2, 3, and 4 illustrate in more detail alternative modifications of the switching apparatus in accordance with the invention which may be used line indicating a two-wire circuit.

as each terminal of the four-wire circuit shown in Fig. 1. a

The diagram of Fig. 1 is not an actual circuit diagram, but rather a single line layout, each A normal make in a circuit is indicated by contacting arrowheads and a normal break in a circuit by separated arrowheads. An arrow directed at a make point indicates that the circuit will be disabled at that point by operation of the associated control device (amplifier-detector) and an arrow directed at a break point indicates that the circuit will be completed thereat by operation of the associated control device.

Fig. 1 shows connected between the two-way circuits L and L1 a four-wire circuit comprising a two-wire path LE adapted for transmission from the circuit L to the circuit L1 in the direction from. west to east and a two-wire path LW arranged for transmission from circuit L1 to circuit L in the direction from east to west. The paths LE and LW are provided with the usual oneway amplifiers AE and AE and AW and AW, respectively. The hybrid coil transformer H1 and associated network N1 connects the input of the path LE and the output of the path LW in substantially conjugate relation with each other and in energy transmitting relation with the twoway circuit L in well known manner. Similarly, the hybrid coil transformer H2 and associated balancing network N2 connects the output of the path LE and the input of the path LW in substantially conjugate relation with each other and in energy transmitting relation with the two-way circuit L1.

Associated with the west end of the four-wire circuit are the amplifier-detectors AD and AD, the inputs of which are respectively connected to the net side of the hybrid coil H1 and to the input of the path LE, and the outputs of which are respectively connected to the winding 11 and b of a differential relay DI having its armature l0 adapted to be engaged with either contact point 26 or contact point 27 associated with relay controls RC and RC, respectively, depending on whether armature I0 is actuated upwardly or downwardly. The energization of the winding a tends to pull the armature of the relay DI downwardly and the energization of the winding b tends to pull the armature upwardly. When the armature ID of the relay DI engages the down contact 26, the relay control RC is operated to actuate switching control RS to disable the path LE, and when the armature l0 engages with the up contact 21, the relay control RC is operated to actuate the switching control TD to disable the relay control RC in its output. When the armature |6 separates from contact 26 the control RC releases and the switching control RS returns to its normal condition in which the path LE is operative, and when the armature I separates from contact 2'! the control RC releases and switching control TD returns to its normal condition in which switching control RC is operative to actuate RS when armature l6 engages contact 20.

Similarly at the east terminal of the four-wire are the amplifier-detectors AD1 and AD1, the inputs of which are respectively connected to the net side of the hybrid coil H2 and to the input of the path LW, and the outputs of which are connected to the windings a1 and b1, respectively of relay D11. The armature l2 of the relay DIl is adapted to be engaged with either contact point 41 or 50 associated with relay controls RC and RC, respectively, depending on whether the armature 2 is actuated upwardly or downwardly. The armature I2 is actuated upwardly or downwardly when the relay winding a1 is energized and downwardly when the relay winding b1 is energized. When the armature |2 engages the upper contact 4i of relay D11 the relay control RC1 operates to actuate the switching control RS1 to disable the path LW, and when the armature |2 engages the lower contact 50 the relay control RC1 is operated to actuate the switching control TD1 to disable the relay control RC1 in its output. When the armature of the relay DI separates from the contact 41 relay control RC1 releases and the switching control RS1 returns to its normal condition in which the path LW is operative. Also, when the armature |2 separates from the contact 50, relay control RC1 releases and the switch control 'ID1 returns to its normal condition, wherein the output of relay control RC1 is operative to actuate switching control RS1 as soon as the relay control RC1 is operated.

Referring to Fig. 2 it will be seen that the amplifier portion of the amplifier-detector AD connected across the net side of the hybrid coil H1 comprises two vacuum tube amplifiers VT]. and VTz (VT1 is a so-called voltage amplifier, while VT2 is a power amplifier) and the detector portion thereof comprises the copper oxide rectifying or detecting unit l6 connected between the output transformer of the amplifier and the winding (1 of the relay DI in series with the resistance H. A condenser i8 is connected across the rectifier l6 and the secondary winding of transformer in series so as to be charged from the output of the rectifier.

The amplifier-detector AD connected across the input of the path LE similarly comprises a vacuum tube voltage amplifier VT1, a vacuum tube power amplifier VT'2, a copper oxide rectifying unit 2| connected between the output transformer 26 of the amplifier and the winding b of relay DI in series with resistance 22. A condenser 23 is connected across the rectifier 2| and the secondary winding of transformer 20 in series so as to be charged from the output of the rectifier.

As in the system of Fig. 1, the armature ID of differential relay DI is arranged to engage either the lower contact point 26 or the upper contact point 2? depending on whether the winding a or the winding b is energized. In the absence of received signal currents, when neither winding is energized, the armature is disengaged from either contact. The contact point 26 is connected by lead 49 through the winding of relay 28' and series resistance 29 to the positive terminal of battery 36; and contact point 2'! is connected by lead 46 through the winding of relay 3| and series resistance 32 to the positive terminal of battery 3|]. A condenser 34 is connected across the circuit comprising the winding of relay 28, the resistance 29 and the battery in series, and the condenser 39 is connected across the winding of relay 3|, the resistance 32 and battery 36 in series.

The armature and contact 35 of relay 28 are normally closed to connect the two secondary windings of the transformer 36 and the two primary windings of transformer 37 of the transformer network in the path LE in series aiding relation so that signals are normally freely transmitted through the network, and are opened in response to energization of the relay 28 to connect those windings of the transformers 36 and 31 in opposing relation so that transmission of signals through the transformer network over the path LE is substantially blocked. The armature and contact 38 of the relay 3| are normally open, and are closed in response to energization of the relay to reconnect the windings of the transformers 36, 3? in series aiding condition so that signal transmission therethrough over the path LE with little attenuation is permitted.

The transformer network 36, 31 and the connections just described for changing it from the normal low loss condition to the high loss condition, or from the high loss condition to the low loss condition, are disclosed and described in H. C. Silent Patent No. 1,749,841, issued March 11, 1930, and, therefore, need not be described in more detail here.

Relays 28 and 3| and their associated circuits as described above comprise relay controls RC and RC, respectively, in the circuit of Fig. 1. The circuits controlled by relays 28 and 3| described above form the switching control RS and the switching control TD, respectively, in the circuit of Fig. l. The condensers and resistances associated with the windings of the relay DI, and the windings of relays 28 and 3| provide the usual slow-release circuits therefor.

As will be apparent from the above description, when the armature of relay 28 engages the contact 35 or the armature of relay 3| engages contact 38, or when both contacts engage their respective armatures, the transformer network 36, 37 (corresponding to switching control RS in the system of Fig. l) 'is conditioned to allow free transmission of signals therethrough over the path LE, and only when the contacts 33 and 38 are simultaneously disengaged from their associated relay armatures is the transformer network 36, 31 (or transmission control RS) conditioned to block signal transmission over the path LE.

Operation of circuit of Fig. 2

In the circuit of Fig. 2, energy impressed on the input of the circuit AD from the net side of hybrid coil H1, will be amplified by the amplifiers VT1 and VTz and rectified by the rectifier I6. The rectified currents will charge up condenser I8 to a value dependent on the input and from then on will cause energizing current to flow through the resistance H and the winding a of the relay DI, so that the relay tends to pull the relay armature l0 toward the lower contact point 26. Similarly, energy impressed on the input of the circuit AD from the path LE, will be amplified by the amplifiers VT1 and VT'z and rectified by the rectifier 22. The rectified currents will charge up condenser 23 to a value dependent on the input and from then on will cause energizing i currentto flow through the resistance 22 and the winding b of the relay DI, thus tending to cause the relay armature III to be pulled toward the upper contact point l1.

When the pull on the armature l0 exerted by winding a prevails over that exerted by winding lathe armature'closes through contact 26 an energizing circuit for the winding of relay 28 from the battery 3!! through resistance 29 and condenser 34 normally charged from battery 30 will discharge. Relay 28 will then operate to separate its armature from contact 35 causing a large disabling loss to be inserted in path LE by transformer network 36, 31 in the manner previously described. When winding b prevails over winding 0, the armature of relay DI closes through contact 21 an energizing circuit for the winding of relay 3| from battery through resistance 32 and condenser 39 normally charged from battery: 38 will be discharged. Relay 3| will then operate to close through contact 38 a current causing the transformer network 36, 3'1 to be conditioned to remove the loss from the path LE, as previously described.

When the output of the rectifier IS in the circuit AD or the output of rectifier 2| in .AD ceases, the associated winding a: or b of the relay DI are maintained. energized for the desired hangover time interval by the discharge of the condenser I8 or 23 through the winding. Also, when an energizing circuit for the winding of relay 28 or 3! is completed through contact point 25 or 21, these relays will be maintained operated for a desired hangover time interval, while condenser 34 or condenser 39 is being charged up by current from battery 30 through resistance 29 or resistance 32 and the relay winding.

Operation of system of Fig. .1

. referring .to Fig. 1.

Let it be assumed that voice currentsfor trans-- mission in the direction from east to west are being received by the four-wire circuit from the two-way circuit L, and at that time no voice signalsuare being receivedby the four-wire circuit over the two-way circuit L1. 1 1

The voice currents from the line L divide in the hybrid coil transformer Hi, the useful part passing. out over the path LE and a portion thereof being impressed on theamplifier-detector AD connected to its input. The received voice currents will have substantially no effect on the amplifier-detector AD connected to the net side of the hybrid coil H1, because of the balance provided between the transmitting and receiving paths LE and LW by network N1.

The received voice currents are amplified and detected in AD and cause winding b of the relay DI to be energized. Relay DI operates to close its armature l9 and contact 21 causing the operation of relay control RC". The latter actuates transmission control TD to disable the out- 1 put of the relay control RC so that the transmission control RS in the path LE remains in its normal condition-permitting transmission of voice signals out over the path LE.

.-The transmitted voice currents at the east terminal of the four-wire circuit are divided by the hybrid coil Hz, the main portion being transmitted into the line L1 over which it is transmitted to a listening subscriber, and substantially all of the remaining portion passing into network N2. The latter voice currents will be impressed on the amplifier-detector AD1 causing energizing current to be supplied to the winding a1 of relay DI. The relay DI]. will then operate to close its armature and contact 41 causing the relay control RC1 to operate to condition switching control RS, to block transmission over the path LW. The echoes of the west subscribers voice currents returned from circuit L1 will be impressed on the amplifier-detector AD'1 connected to path LW causing energization of winding In of relay DI1 in its output. The relationship between the amplifier-detector AD1 and associated circuit and the amplifierdetector AD'1 and the associated circuit is made such by suitable design that the pull on the armature of relay DI1 exerted by winding d1 due to the voice signal impressed on AD1 is always sufficiently greater than the pull exerted by the winding In due to the echoes impressed on AD1 as to hold the relay armature on contact 41. 'Thus, the transmission control RS1 in path LW will remain in condition to block transmission of echoes over that path;

The relationship between the switching circuits associated, respectively, with the outgoing one-way path and the net side of the hybrid coil at each terminal may be expressed as follows:

SN=SL- (4L+R) Where SN=Sensitivity of amplifier-detector bridged on the net side of hybrid coil SL=Sensitivity of amplifier-detector bridged on the outgoing one-way path of the four-wire circuit R=Lowest return loss of the associated twoway circuit L Loss of the hybrid coil Thus, the portion of the west subscribers voice energy which affects the amplifier-detector AD1 after arriving at the hybrid coil H2, is attenuated by the loss across the hybrid coil plus the return loss of the two-way line L1 while that portion which affects amplifier-detector AD1 is attenuated by only the loss from the four-Wire to the two-wire side of the hybrid coil H2. As long as the proper sensitivity relationship between amplifier-detectors AD1 and AD'1 is maintained, the influence exerted on amplifier-detector AD1 due to incoming energy over path LE will always be more efiective than that exerted on amplifier-detector AD1 solely by echo energy from L1. Therefore, relay DI1 will always operate upwardly as a result of the energization of its winding (11 when the party connected to L1 is silent.

Now, let it be assumed that the east subscriber connected to line L1 commences to talk while the west subscriber is talking. The formers voice currents'will be divided at hybrid coil Hz, the

tact point 41 to contact point 50. Relay control RCi then operates causing control TD to disable relay control RC1 in its output, thereby returning control RS1 to its normal condition so as to allow the voice energy of the east subscriber as well as the echoes of the voice energy of the west subscriber to be transmitted over line LW. Accordingly, the voice energy of the east subscriber will eventually reach the west subscriber associated with line L and thus will inform the latter that the east subscriber Wants to break in.

It will be noted that the echo suppressor arrangement described above distinguishes from prior art terminal echo suppressors in two important respects; namely, first, the listening subscriber is not required to await a pause in the transmission of the voice energy of the talking subscriber before breaking in, the circuit conditions requiring only that the wave energy of the former be efiectively stronger at the terminal of the four-wire circuit at the listeners end than that of the latter; second, that except in the presence of actual double talking neither party will hear an echo unless the speech is so weak that it fails to operate the proper amplifier-detector. Since echoes can be heard only during actual double talking, the former will be largely masked by the direct speech. In the present circuit arrangement, it is not possible for incoming speech to operate any control other than RS or RS, as the case may be.

Referring to Figs. 1 and 2, let it be assumed that the east subscriber has been talking, and pauses momentarily or stops talking. The relay DI will not immediately release to break the connection between its armature and contact 29, for a current is caused to flow for a brief interval through resistance I? and winding of relay DI due to the release of the charge that was stored in condenser I8 when current was initially passed through winding :1. By a proper proportioning of the elements of the discharge circuit current of suitable value may be maintained in coil a for a sufiicient hangover interval of time so that delayed echoes of the east subscribers speech returning from the twoway line L and causing the energization of winding b of the relay cannot cause false operation of that relay to operate relay 3I.

Also, in the arrangement shown in Fig. 2, it is apparent that as soon as incoming transmission over path LW ceases, condenser I8 commences to discharge through coil a. of relay DI, and, thus, the sensitivity of amplifier-detector AD must be made low enough so that voice energy returning as echoes from line L and impressed on AD so as to cause energizing current to be supplied to winding coil b of relay DI will not be sufficient to enable winding 1) to prevail over winding coil (1. as a result of partial discharge of condenser I8 in AD. Obviously, if the correct value of current could be maintained in winding a of relay DI for a time interval equal to the delay of the circuit connected to the two-Way line L and, then, is terminated rather abruptly, the above arrangements would be somewhat improved, otherwise a considerably larger charge must be initially built up in condenser I8 so that there will be sufficient charge remaining when the delayed echoes return. In the former case, the sensitivity of amplifier-detector AD would then be limited only by the loss in the echo path as previously described. Furthermore, there would be no biasing action, occasioned by the trailing out of the discharge of condenser I 8 against the operation of relay DI by winding 2) except during the period when the biasing action was actually essential.

Fig. 3 illustrates a modification of the arrangement disclosed in Fig. 2. As in the case of Fig. 2, the apparatus of Fig. 3 would be employed both at the west and east ends of the four-wire transmission circuit represented in Fig. 1. Identical parts appearing in Figs. 1 and 3 are designated by. the same reference numeral.

Connected across the circuit intermediate the hybrid coil H and network N1 is the input of an amplifier comprising the tandem amplifier tubes V1 and V2. In the output of the latter is a transformer 51 having one side of its secondary Winding connected through a copper-oxide rectifying or detecting unit 58 and primary winding of coil 59 to the grid of the vacuum tube I9, and through rectifier 58, primary winding of coil 59 and normally closed contact 6| of charge maintainer relay CM to the grid of tube 65. A condenser 66 is connected across the grid-filament circuit of tube 65. The other side of the secondary winding of transformer 51 is connected to the filaments of tubes 65 and I9. The plate circuit of tube 65 includes in series battery 10, winding 0 of relay DI and winding a of magnitude indicator relay MI. Condenser I4 is connected across the secondary winding of transformer 51 and the rectifying unit 58 in series. Also, a resistance I5 is connected across the secondary winding of transformer I5, rectifier 59- and the primary winding of coil 59 in series.

The plate circuit of tube 19 includes in series plate battery 83, winding 17 of relay MI and resistance 85. The secondary winding of coil 59 is connected to the Winding of fall indicator relay FI which is arranged to operate only on a decrease in the current from the rectifying unit 58, and to remain inoperative on an increase of current from the same source. The armature 9| of relay FI is connected to ground and through condenser 94 to the normally open contact point 92 of the relay FI. The contact point 92 is connected through the winding of relay CM to the armature 93 of relay MI, and also through resistance 95 to the normally open contact 98 of relay MI and to the grounded battery 96.

Connected across the input of the path LE is the input of the control circuit including amplifiers VT1 and VT'z. In the output of the latter is a transformer I05 having one side of its secondary winding connected through a copper-oxide rectifying or detecting unit I06 and grid battery I01 to the grid of tube I08, and the other side connected to the filament of tube I08. The plate circuit of tube I08 includes in series plate battery I09, winding b of relay DI and resistance II2. A circuit comprising the resistance I02 and parallel condenser I03 is connected across the secondary winding of transformer I05 and the rectifying unit I06 in series.

The grounded armature 99 of relay DI is arranged to engage either contact point III or contact point II8, depending on whether the winding b or a is energized. Contact point In is connected through the winding of relay H9 and series resistance I20 to grounded battery I2I; and contact point H8 is connected through the winding of relay I22 and series resistance I23 to the grounded battery I2I. Contact I21 of relay I22 is normally closed to connect the two secondary windings of transformer I28 and the two primary windings of transformer I29 to enable a transmission of voice energy therethrough over the path LE, and when opened by operation of relay I22 connects these same windings in opposing relation so that the transformer network prevents transmission over the path LE. Contact I39 of relay II9 is normally open and. is closed by operation of relay H9 in parallel with contact I21 and the armature of relay I22, to

prevent relay I22 from disabling the path LE. Condenser I 3| is connected across the circuit comprising the winding of relay I22, resistance I23 and battery I2I in series. Condenser I33 is connected across the circuit comprising the winding of relay H9, resistance I29 and battery I2I in series. The resistances and condensers associated respectively with relays H9 and I22 pro vide the usual slow-release circuits therefor.

Operation of circuit of Fig. 3

Voice currents originating with the east talking subscriber connected to two-wire line L1, as described with respect to Figs. 1 and 2, and incoming over line LW are divided in hybrid coil H1 in the usual way, that is, part of the energy continues on to the subscriber connected to two-wire .line- L, and another part passes into network N1.

In the latter case, the voice energy is amplified by amplifiers VTl and VTz, and is rectified by rectifying unit 58. The rectified current immediately charges condensers I l and 69, and at the same time, provides an equal voltage across resistance I5. As long as the input to rectifier 58 continues to increase, the charge on condenser 66 will continue to increase to place a more positive charge ,on the grids of tubes I55 and I9, thereby eifecting corresponding increases in their plate current. The winding a of relay D1 in the plate circuit of tube 65 is made equal in resistance value to resistance 85 in the plate circuit of tube I9. The tubes 65 and I9 are of a similar ,type, and, are provided with identical grid batteries and plate batteries I9 and 85. As long as the voltages on the grids of tubes 65 and 79 are equal, relay MI will be unable to operate in either direction. However, as soon as the energy flowing through winding .0. of relay DI is sufficient to effect its operation, relay DI will operate to close armature 99 and contact point H8, thus completing an energization circuit for relay I22 which may be traced from ground through armature 99 and contact point H8 associated with operated relay DI, winding of relay I22, resistance I23 and battery I2I to ground. Relay I22 operates to open contact I2I to break the bridge connection on transformer network I28, I29, so that it blocks transmission over the path LE.

Voice currents returned as echoes from the twowire lineL enter the path LE and are impressed on the associated control circuit. The impressed echoes are amplified by amplifiers VT'1 and VT2 and are then rectified by rectifying unit I96. The rectifier output charges condenser I93 and, thereafter, makes the grid of tube I98 more positive causing plate current to fiow through winding b of relay DI, tending to operate the latter. The relative sensitivities of amplifiers VT1VT2 and VT1VT'2 are made such thatthe amount of current flowing through the a winding of relay DI due to incoming speech waves is always larger than that flowing through the b winding of the relay due to echoes of that speech only. Under these conditions the armature of the relay DI will always be operated to contact H8 and the transformer networklZB, I29 will be conditioned, as previously described, to its high loss condition so as to suppress echoes in the path LE.

As soon as the Voice energy incoming over the path LW commences to decrease relay FI operates to close its armature SI and contact point 92, thereby effecting a discharge of condenser 94 to ground, and, in addition, completing an energization for relay CM from battery 96. This circuit may be traced from ground through armature SI and contact point 92 of operated relay FI, winding of relay CM, resistance 95, battery 96 to ground. Relay CM then operates to open contact 6! to disconnect condenser 66 from resistance I5. As a result of this the condenser 66 holds its charge and, therefore, maintains the value of current then existing in winding a; of relay DI and winding a of relay M1 at the instant immediately prior to the operation of relay CM. If no additional voice currents reach amplifier VTi within a given interval of time, relay CM will eventually release due to the fact that condenser 94 will be charged up through resistance 95 and the winding of relay CM from battery 99. The release of relay CM closes contact El causing condenser 66 to be rapidly discharged through resistance I5 with the result that the value of current in winding 01. of relay DI will quickly fall to zero breaking contact H8. A short time later after condenser I3! has been completely charged by battery I2I, through resistance I23 and the winding of relay I22, the current in relay I22 falls to zero and that relay will release thereby returning the relay contact IZI to the normally closed condition in which the transformer network I28, I29 is in the low loss condition, so that the path LE is in transmitting condition.

However, if Voice currents incoming from line LW and being of a value larger than the maximum received heretofore are impressed on amplifier VT1 before relay CM releases, they will eventually eifect a voltage across resistance III which will cause the plate current of tube I9 to reach a larger value than the plate current of tube 65, thereby producing a larger current in winding 1) of relay MI than that in its companion winding a. Relay MI then operates to its contact 93 shortcircuiting resistance 95 and causing condenser 94 to charge up quickly from battery 26, through contact 93 and armature 93 of operated relay MI, and the winding of relay CM. When condenser 94 is charged, current ceases to flow through relay CM so that it releases to close contact 6!, thereby enabling condenser 69 to charge up to the proper value in accordance with the larger amount of voice currents being impressed on amplifier VTl. Thereafter, the echo suppressor ap paratus will function as previously described.

While the subscriber associated with the east terminal is talking and the suppressor circuits at the west terminal are conditioned in the manner which has just been described, the west subscriber can break in instantly by talking. His voice currents will be impressed on the control circuit connected to the path LE and after being amplified by the amplifiers VT'1 and VT'z and rectified by the rectifier I96 therein will produce sufiicient flow of plate current in the output of tube I99 and through the b winding of relay DI as to make that winding prevail over the a. winding. The armature of relay DI will then close through contact III an energizing circuit for the winding of relay II9 from battery I2I through resistance I29 and the latter relay will operate to close contact I30. This will, in the manner which has been previously described, connect the windings of the transformer net work I28, I 21 so that transmission through that network over the path LE may take place.

Fig. 4 illustrates a modified terminal echo suppressor arrangement in which either subscriber can remove the echo suppressor and break in on the other subscriber even when the latter has not ceased talking for a time interval of sufficient duration to allow a release of his echo suppressor apparatus. In Fig. l, between twowire line L and two-wire line L1, there is a four-wire circuit having a path LE adapted for transmission from line L to line L1, and a path LW adapted for transmission from line L1 to line L. Transmission paths LE and LW are provided with conventional one-way amplifiers AE and AE, and AW and AW, respectively. At the west terminal of the four-wire circuit, a hybrid coil H1 connects the two-Way line L to the paths LE and LW, and a network N balances the line L. Likewise, at the right terminal of the four-wire circuit, a hybrid coil m connects two-way line L1 to the paths LE and LW, and a network N2 balances line. L1. The broken portions of paths LE and LW represent long intermediate sections of the four-wire system which may be of any desired length.

The echo suppressor arrangement associated with the west terminal of the four-wire circuit includes amplifier A and full wave copperoxide rectifying unit RU connected to the outgoing path LE, and an amplifier A1 and full wave copper-oxide rectifying unit RU1 connected to the incoming path WE, the former being responsive to outgoing waves in the path LE to supply energizing current to the winding a of the differential relay DI, and the latter being responsive to incoming waves in the path LW to supply energizing current to the winding 1) of the same relay. The relay DI controls the operation of the suppressor relay RS which, in turn, controls the loss condition in the path LE in the manner which will be described later in connection with the description of operation of the whole system. A similar echo suppressor arrangement is located at the east terminal of the four-wire circuit for controlling the loss condition of the outgoing path LW thereat. The elements in the latter arrangement bear the same reference characters as the corresponding elements in the echo suppressor arrangement at the west terminal, but followed by a prime mark.

Operation of system of Fig. 4

The operation of the system of Fig. 4 is as follows:

Let it be assumed that the west subscriber .associated with the two-way line L starts to talk and the east subscriber associated with the east two-way line L1 is silent. Wests voice currents will be divided in the hybrid coil H1, the useful portion entering the input of path LE and the other portion entering the output of path LW and eventually being dissipated in the output of the amplifier AW therein. The voice waves in the path LE will be amplified by the amplifier AE and the main portion thereof will be transmitted out over the path LE towards the east terminal.

A small portion of wests waves will be diverted into the control circuit connected near the west terminal, and will be amplified by the amplifier A and rectified by the rectifier RU therein. The rectified currents will energize winding 0. of the differential relay causing its operation to pull its armature I49 upwards. Meanwhile, the portion of Wests voice waves entering the output of path LW will be impressed on the associated control circuit, will be amplified by amplifier A1 and rectified by rectifier RU1 therein and supplied as energizing current to the winding b of relay DI and tend to cause armature I 49 to be pulled in the downward direction. Because of the loss encountered in the hybrid coil H1 and in loss pad I45 in the output of LW the latter energizing current will be so weak as not to have much efiect on the operation of the relay DI.

With the armature of relay DI pulled upward, the energizing circuit of the relay RS is broken so that that relay will remain in its normal unoperated condition wherein the loss condition in the path LE is low. The path LE is then operative to transmit freely Wests voice currents.

Eventually Wests voice currents reach the east end of the four-wire circuit, the main portion passing through the loss pad I45 and being impressed by the hybrid coil H2 on the twoway line L1 and being transmitted out over that line to the east subscriber. A portion of the received (wests) voice waves are diverted from the out put of LE into the associated control circuit wherein they are amplified by amplifier A, are rectified by rectifier RU and supplied as energizing current to the a winding of the differential relay DI.

The condenser connected across each winding of the difierential relay DI, and those connected across each winding of the relay DI at the west terminal also are provided to by-pass the alternating current components of the rectified waves.

Relay DI then operates to pull its armature I5I upward making contact with contact point I52 and completing an energization current for the relay RS from battery I 51. This circuit may be traced from ground through armature I5I and contact point I52 of operated relay DI, series resistance I54, winding of relay RS, resistance I56 and battery I51 to ground. Relay RS then operates to close contact I58, thereby short-circuiting path LW and blocking transmission thereover.

A portion of the voice current passing into the two-wire line L1 will be returned as echo to the path LW and will be impressed on the control circuit connected to its input. These Waves will be amplified by amplifier A'2, rectified by rectifying unit RU1 and applied as energizing current to winding 1) of relay DI. The energized winding b will tend to operate the armature I5I of relay DI in a downward direction and opposite to that effected by energized winding 11. However, the sensitivity of receiving amplifier A and rectifying unit RU is made sufficiently greater for the received voice currents than that of transmitting amplifier A1 and rectifying unit RU1 for echo energy returning from line L1 so that the relay DI will be maintained actuated upwardly to contact I52. For this reason, therefore, it is readily apparent that false operation of relay DI by echoes of wests incoming speech transmitted through the coil is entirely prevented In case delayed echo arrives after the direct incoming speech impulse has ceased, it may operate the armature I5I of DI relay in the down direction momentarily but due to the slow release circuit associated with relay RS as described below, this echo will still be suppressed.

W'hen relay DI operates due to the energization of its winding a, condenser I62 normally charged by battery I5'I discharges through resistance I54, contact point I52 and armature I SI of after a brief interval.

2,093,525 operated relay DI to ground, the condenser I62 and resistance I54 forming a slow release circuit for relay RS. Accordingly, relay RS remains operated for a short hangover interval after relay DI may have released, thereby blocking any echoes due to trailing weak parts of speech and delayed echoes from the line L1. As indicated a delay circuit DC may be inserted in the path LW in front of the disabling point to store initial echo waves until relay RS has had sufiicient time to operate. A delay circuit DC may be inserted in the path LE at the other terminal for this purpose for speech transmission in the opposite direction.

Let it now be assumed that the east subscriber connected to the two-wire line L1 wishes to break in while the west subscriber connected to two-wire line L is talking. The former may commence to talk while the latters speech currents are still being received, it being unnecessary for him to await a pause in the speech incoming over line LE. It is only required that easts speech wave at the output of the amplifier A'1 bridging the transmitting path LW be efiectively stronger than wests at the output of amplifier A bridging the receiving path LE. This may be accomplished since the transmitting bridging amplifier A'1 and rectifying unit RU'1 may be made considerably more sensitive than receiving bridging amplifier A and rectifying unit RU without causing false operation of relay DI due to echo.

When easts voice currents have caused operation of relay DI in the down direction due to energization of its winding 1), relay RS previously operated by As speech will subsequently release After the energization circuit for relay BS is broken at contact I52, the relay BS is held operated for this brief hangover interval until condenser I62 is completely charged by battery I51 through the winding of relay RS and resistance I56, and it then releases.

A portion of easts voice currents arriving at the west terminal will be picked up by receiving bridging amplifier A1 and another portion will pass through hybrid coil'I-I out over line L to the west subscriber. Accordingly, the west subscriber will hear the east subscriber. The relative sensitivities of the two circuits controlling the DI relay at the west terminal is such that easts speech through amplifier A1 and rectifier RUi will not be able to operate the DI relay unless west has stopped talking or is talking considerably weaker than west, so that the energizing current in the winding 1) of relay'DI is sufficiently greater than that in winding (1. Of course, echo will occur during the. period of double talking. As soon as A ceases talking the echo suppressors will operate in the manner previously described to suppress the echo.

The sensitivities of the receiving bridging amplifier A and rectifying unit RU at the east end of the four-wire circuit and thatof the receiving bridging amplifier A1 and rectifying unit RU1 at the west end of the four-wire circuit are limited by incoming noises in the usual manner, that is, each of these control circuits must be adjusted so that it will 'not operate on noise since such a happening would result even under normal conditions in considerable mutilation of the outgoing talkers voice waves. The sensitivities of the transmitting bridging amplifier A and rectifying unit RU at the west terminal and of the transmitting amplifier A1 and rectifying unit RU'1 at the east terminal may be made greater than those of the associated receiving control circuit up to a point where they just prevent an operation of the relay DI or DI skipped lines in the transmitting direction, the sensitivity will be proportionately lowered. Obviously, the subscriber at the opposite end of the four-wire circuit will then fail to operate the disabling relay BS or RS unless his speech wave create a stronger pull on relay DI or DI than the noise. As previously explained, echo from incoming speech never causes as strong a pull on the trans mitting side of the DI or DI relay as the incoming speech waves doon the receiving side. Therefore, it follows that echoes of incoming speech waves which failed to overcome the effect of noise will usually be weaker than the noise. Further, experiments have shown that echoes which are equal to or less than the noise currents are not very objectionable.

It is to be understood that the invention is not limited to the precise details of the circuits illustrated and described for it is apparent that numerous modifications thereof could be made for those skilled in the art within the spirit and scope of the invention.

What is claimed is:

1. In combination in a two-way signal wave transmission system, a four-wire circuit for repeating signals in opposite directions, and means for directionally controlling transmission in said circuit comprising near each terminal of said circuit means for effectively balancing the transmission in the outgoing side against that in the incoming side of said circuit, said. balancing means being responsive to a higher transmission level in said incoming side than in said outgoing side to'cause said outgoing side to be disabled, and

responsive to a higher transmission level in said outgoing side than in said incoming side to prevent said outgoing side from being disabled, and

slow-releasing after operation in either direction.

3. In a two-way signaling system, two twoway lines, a four-wire circuit comprising oppo sitely-directed one-way paths connecting the two-way lines, and an echo suppressor unit associated with the paths near each terminal of the four-wire circuit comprising difierential means which when operated in one direction disables the transmitting path and when operated in the opposite direction prevents the transmitting path from being disabled, and control circuits connecting the differential means to the transmitting and receiving paths for effecting operation of the differential means, the sensitivities of the control circuits being relatively adjusted so that the differential means will always be operated in said opposite direction in response to signals outgoing in the transmitting path irrespective of a simultaneous presence of signals incoming in the receiving path, and means associated with said control circuits for making said diiTerential means slow releasing after operation in either direction.

4. In a signaling system, two two-way lines, a four-wire circuit comprising one-way paths for transmitting signals in opposite directions between the two-way lines, and an echo suppressor unit associated with the paths near each terminal of the four-wire circuit comprising electromagnetic means for disabling the transmitting path, a second electromagnetic means for preventing a disabling of the transmitting path, differential means which when operated in one direction effects an operation of the first-mentioned electromagnetic means and when operated in an opposite direction effects an operation of said second electromagnetic means, and control circuits respectively connected to the transmitting and receiving paths, for effecting operation of the differential means, the sensitivities of the control circuits being relatively adjusted so that the differential means will be operated in said opposite direction when signals are outgoing in the transmitting path regardless of a simultaneous existence of signals incoming in the receiving path.

5. In a two-way signaling system, two twoway lines, a four-wire circuit comprising oppositely-directed one-way paths connecting the two-way lines, and an echo suppressor unit associated with the paths relatively near each end of the four-wire circuit, comprising differential means which when operated in one direction disables the transmitting path and when operated in the opposite direction prevents the transmit ting path from being disabled, control circuits connecting said differential means to the transmitting and receiving paths in a manner tending to operate the differential means in one direction in response to signals incoming in the receiving path and in the opposite direction in response to signals outgoing in the transmitting path, the sensitivities of the control circuits being relatively adjusted so that the differential means will be operated in said opposite direction by outgoing signals received in the transmitting path from its associated two-way line whether or not incoming signals are simultaneously present in the receiving path, and auxiliary means for preventing false operation of the differential means by signal echoes during pauses in signal transmission between the two-way lines.

6. The system of claim 5, in which said differential means comprises a differential relay and said auxiliary means comprises means making said relay slow releasing to the required degree.

'7. The system of claim 5, in which said differential means comprises a relay having two opposing windings, said control circuits comprise two amplifier-rectifiers, respectively, connected to the transmitting and receiving paths and controlling the energization of a different one of said windings, and said auxiliary means comprises a capacity-resistance arrangement associated with each Winding for making said relay slow releasing to the required degree for each direction of operation.

8. The system of claim 5, in which said auxiliary means includes a condenser which is charged up when said differential means is operated and while charged maintains said differential means operated, and means for causing said relay to suddenly discharge a desired time interval after cessation in the supply of controlling signal energy to said differential means.

9. In a two-way signaling system, two two-way lines, a four-wire circuit comprising oppositelydirected one-way paths connecting the two-way lines, and an echo suppressor unit associated with the transmitting and receiving paths near each end of the four-wire circuit, comprising electromagnetic means for disabling the transmitting path, a second electromagnetic means for preventing a disabling of the transmitting path, a mechanical relay having two opposing windings whereby an energization of one winding effects an operation of the first electromagnetic means and an energization of the second winding effects an operation of the second electromagnetic means, amplifier-rectifier units having their outputs connected to a different one of the windings and their inputs respectively connected to the transmitting and receiving path so that said one winding is energized in response to signals incoming in the receiving path tending to operate said first electromagnetic means and said second winding is energized in response to signals outgoing in the transmitting path tending to operate said second electromagnetic means, the sensitivities of the amplifier-detector units being relatively adjusted so that the relay will effect an operation of said second electromagnetic means when outgoing signals are present in the transmitting path irrespective of the simultaneous existence of signals incoming in the re ceiving path, and a capacity-resistance means connected between the outputs of said amplifierrectifier units and the windings of the relay for rendering the relay slow releasing after its operation to prevent false operation thereof by signal echoes during pauses in signal transmission between said two-way lines.

10. In a two-way signaling system, a four-wire circuit comprising one-way paths for transmitting signals in opposite directions between two two-way lines, and an echo suppressor unit near each terminal of the circuit, comprising differential means which when operated in one direction disables the outgoing path thereat and when operated in the opposite direction prevents said outgoing path from being disabled, control circuits respectively connected to the outgoing and incoming paths at the terminal and controlling the operation of said differential means, the sensitivities of the two control circuits being relatively adjusted so that said differential means will always be operated in said opposite direction when outgoing signals are present in the outgoing path, and means for preventing false operation of said differential means by signal echoes dun'ng pauses in signal transmission comprising a condenser which is charged up in response to incoming signals and when the differential means is operated in said one direction maintains it so operated, and a mechanical relay responsive to cessation in the incoming signals to maintain said condenser charged for a suitable interval of time thereafter and then to cause said condenser to discharge suddenly.

DOREN MITCHELL. 

